While conventional computers store data as bits on memories, such as RAM, quantum computers store the data as qubits, that is, quantum states in a two-state quantum-mechanical system. A variety of quantum-mechanical systems may be used, such as the polarisation of a single photon, electron spin and others.
A common problem with quantum-mechanical systems is that the quantum system inevitably interacts with the environment in an uncontrolled fashion. Any fluctuations in the environment can be seen as noise that causes the quantum state to deteriorate. This is also referred to as decoherence. In order to build quantum technology, such as a quantum computer, it is necessary to store qubits similar to registers in conventional processors. However, the native coherence time of the quantum state (useful lifetime of the quantum state) is too short for most practical applications, which means that the quantum state is not recoverable in a useful form when it is needed for the next step in the computation process. Even small accumulation of error probability may render a quantum state useless for a specific purpose.
There is a need for technology that preserves the quantum state such that it is accessible for quantum information systems. This would be required, for instance, in a quantum repeater, or in a quantum computer where some quantum information must be maintained with error rates deep below fault-tolerant thresholds while large blocks of an algorithm are carried out on other qubits.
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